The invention relates to a method for cleaning exhaust gases from a combustion engine, in which method humid air is fed into a combustion chamber of the engine, whereby a mixture of gas and aqueous mist is fed with a spray head into a space that leads to the combustion chamber.
The invention also relates to an apparatus for feeding humid air into a combustion engine, the apparatus comprising a spray head for feeding aqueous mist into a space in the combustion engine, gas feeding means for feeding gas into the spray head, liquid feeding means for feeding aqueous liquid into the spray head and mixing means for providing a mixture of gas and aqueous mist from the spray head into said space, which is arranged to be in flow connection with the combustion chamber of the combustion engine and to supply the mixture of gas and aqueous mist into the combustion chamber. This kind of apparatus is known in connection with gas turbine engines, and the purpose of the apparatus is to prevent power and efficiency of the engine from declining when the temperature is high around the turbine, as is the case on hot days.
One aim of environmental protection is to reduce exhaust gas emissions from combustion engines. The present invention provides a solution, by which exhaust gas emissions from diesel engines, in particular, but also from other combustion engines will be reduced.
Exhaust gases from diesel engines contain various harmful combustion products, of which oxides of nitrogen, i.e. NOx, are the most harmful to the environment. The oxides of nitrogen considerably contribute to smog formation, green house effect and soil acidification as well as retard the growth of forests, for instance.
The diesel engines of ships are great polluters of air. According to an American study, published a few years ago, 14% of the nitrogen emissions in the world and 16% of the sulphur emissions originate from marine traffic. Soot/carbon emissions from the diesel engines, which are produced most when the engine runs at a low power level, also pose a problem. Considerable nitrogen emissions are also produced when the engine runs at a low power level. Typically, ship engines are run at a low power level when the ships are in ports, so the emissions are a considerable problem also when the ship is in port.
In diesel engines, nitrogen emissions can be reduced in a known manner by lowering the combustion temperature, whereby emissions are reduced while being produced. The combustion temperature can be lowered in a variety of ways. It can be lowered by injecting water into the combustion chamber or by using an aqueous emulsion in fuel.
According to some studies, injecting water into the combustion chamber may increase smoke emissions. Fuel consumption may also rise, if large amounts of water are injected into the engine. However, other studies show that the so-called HAM (Humid Air Motor) method, in which humid air is fed into the motor, yields good results regarding emissions. In this method the charge air of the diesel engine is humidified by an evaporator and the fuel burns in the engine cylinders in humid air instead of normal air. The HAM method has a drawback that the load capacity of the diesel engine is considerably reduced. A further drawback of the method is that water cannot be dispersed into sufficiently small drops in the evaporator, but the drops produced therein are relatively large, and consequently they do not vaporize quickly and readily. Quick vaporization of the drops is a prerequisite for lowering the emissions and making the engine run smoothly also in other respects.
The use of an aqueous emulsion in fuel reduces nitrogen oxides without that the amounts of carbon dioxides would increase. According to some studies, the obtained results are not so good as those obtained by a method, in which water is injected into the cylinders. However, a problem with the “aqueous emulsion method” is that a sufficient amount of water cannot be mixed with the fuel.
U.S. Pat. Nos. 4,459,943 and 4,411,224 disclose systems for feeding gas and water mist into the intake air of a combustion engine. The amount of gas and water mist are each increased directly proportionally to the load of the engine.
An alternative and also complementary method to air humidification is to remove exhaust gas nitrogen emissions with a catalytic converter. With the catalytic converter the oxides of nitrogen are reduced to nitrogen and water vapour by spraying a mixture of urea and water into the exhaust gases. The catalytic converters reduce nitrogen emissions efficiently but in naval applications they are very expensive: they cost about 30% of the engine price and in large engines even more than that. Moreover, in a ship the catalytic converter takes a lot of space and considerable operating costs are incurred from servicing, etc.